在图形化蓝宝石衬底生长低温缓冲层之前, 通入少量三甲基镓(TMGa)和大量氨气进行短时间的高温预生长, 通过改变TMGa流量制备了4个蓝光LED样品。MOCVD外延生长时使用激光干涉仪实时监测薄膜反射率, 外延片使用高分辨率X射线衍射(002)面和(102)面摇摆曲线估算位错密度, 并使用光致发光谱表征发光性能, 制备成芯片后测试了正向电压和输出光功率。结果表明, 高温预生长可促进薄膜的横向外延, 使得三维岛状GaN晶粒在较小的薄膜厚度内实现岛间合并, 有利于降低位错密度, 提高外延薄膜质量, LED芯片的输出光功率的增强幅度达29.1%, 而电学性能无恶化迹象; 但高温预生长工艺中TMGa的流量应适当控制, 过量的TMGa导致GaN晶粒过大, 将延长岛间合并时间, 降低晶体质量。

Before the epitaxy of low-temperature buffer layer, the patterned sapphire substrate was prepared by a pre-growth treatment at a high temperature under a small amount of trimethyl gallium (TMGa) and a great quantity of ammonia (NH3). Four types of blue light-emitting diode (LED) wafers were fabricated using different flow-rate of TMGa. The laser interferometer in MOCVD was used to monitor the thin-film reflectance during the epitaxial growth process. The high-resolution X-ray diffraction rocking curves of (002) and (102) crystal faces were utilized to estimate the threading dislocation density. The photoluminescence spectra were measured to feature the light-output performance. Moreover, LED chips were fabricated and tested. It is shown that the pre-growth treatment benefits the lateral growth of three-dimensional GaN islands in epitaxy process and leads to fast coalescence within a small thickness. Thus, the threading dislocation can be suppressed and the quality of epitaxial film would be improved. The enhancement of light-output power of LED chips can be up to 29.1% without degradation of electrical performance. However, the flow rate of TMGa should be tuned carefully, because excess quantity of TMGa would cause a large size of GaN grain crystal, which induces delayed coalescence time and low-quality of epitaxial film.